The present invention relates generally to valve type voltage arrester devices and in particular to such devices for use with low-voltage installations, and is concerned with that type of arrester device which includes a switching device and a spark gap.
Such arrester devices have been used to a considerable extent to protect domestic electric devices, in particular on overhead supply lines leading to individual buildings, from transient or persistent overvoltages. Valve type arrester devices are however now being increasingly used in those houses whose electricity supply is by underground cable because it has been found that overvoltages, that is to say voltage surges, can occur in supply systems which use only underground cables as well as those which use a combination of underground cables and overhead lines. Furthermore, there is an increasing usage in the household of electrical devices which are particularly sensitive to overvoltages.
For the protection of consumer installations, indoor mounted arrester devices are conveniently used which permit a simple connection to the electrical supply system of the house. They can be clamped or arranged in rows on carrier tracks in a manner similar to protective circuit breakers.
Low voltage valve type arrester devices are indispensable with office buildings which have electrical installations, hospitals, lighting towers and with installations which are protected by earth leakage circuit breakers. Undesired switching of the breakers, which can be triggered by overvoltages, can thereby be substantially avoided. However, the protective ability of previously known low voltage valve type arrester devices is only partial. This is due to the fact that the trigger voltage, that is to say the voltage at which the arresting function is initiated, is set considerably higher than the supply voltage with the result that only large voltage peaks are kept out of the domestic supply circuits. The reason that the trigger voltage is set high is due to the construction of the spark gap with which such arrester devices are usually provided, and in particular to the large tolerances of the components which are used in its construction. The spark gap conventionally comprises two electrodes between which is a dielectric material which is subjected to external influences without hindrance with the result that changes in the ambient temperature, atmospheric pressure or air humidity can change the trigger voltage. It must, therefore, be ensured that even with unfavorable atmospheric conditions, the spark gap does not break down at the normal supply voltage but only when an overvoltage is present. Thus under normal conditions the trigger voltage lies so far above the normal or supply voltage that the protective action, for instance for electronic devices, is no longer adequate.
The protective function of the known low voltage value type arrester device is, however, not only imperfect in the lower region of voltage surges but also can not be relied on at high current densities. It can happen that the cut-off or switching device is unable to switch off extreme short-circuit currents but instead an arc forms between the separating switch elements which does not extinguish itself despite the fact that the distance between the elements is increasing during the switching operation. Thus very high short-circuit currents can be led away but on the other hand the imperfect switching means that there is a substantial safety risk.
Accordingly it is an object of the present invention to provide a valve type arrester device, in particular for low voltage installations, whose protective function is fully ensured with a trigger voltage which is slightly above the supply voltage and even with extremely high short-circuit currents.